Mystery Problem

[mridenour]

I recently bought a 93 Ranger 4x4 Supercab with a 4.0. Put in new plugs, wires, air filter, cleaned the MAF, replaced the TPS, replaced several damaged vacuum lines, new fuel filter, I can't even think of everything I've done at the moment but the truck for the most part runs well. When I first start it up cold it doesn't respond well to acceleration and just runs rough for a minute or two and then it is fine. Then when I get out on the highway I am cruising along at 60 or 65 and it suddenly loses power, I let off the gas for a few seconds and then the power comes back. I am wondering if maybe this could be the fuel pump or something else like the EGR. I have good vacuum on the engine and I think I have got all the vacuum issues solved.

 

[Earl43P]

Try turning the key to the Run position (don't crank the starter)
wait 2 seconds as the fuel pump hum
turn key to Off, repeat
THEN crank it over.

If that helps the cold start problem my guess is that your fuel pump's checkvalve is letting pressure bleed off overnight.

That same fuel pump may not be able to provide enough flow at highway speeds.

A fuel pressure gage may help the diagnosis.

 

[mridenour]

Yeah, I definitely need a gauge. I just spent a bunch of money replacing the whole brake system but I'll have to buy another tool. The gauge I have is from carb days...lol!

 

[Earl43P]

Without a gage, wait till it "dies" at speed.
Turn the key to Off and coast to the side. (obviously don't lock the steering wheel)

Depress the schraeder valve and see if fuel pressure is present.
Restart, drive away with your diagnosis.

If you have no fuel pressure after it has died, it could be bad FP relay.
Every bad fuel pump I've seen just plain doesn't pump (ever) with power applied, not intermittent like you describe.

I'd even take the step of temporarily jumpering the inertia switch (use one scotchlock to join both wires for a day or two) to see if IT is intermittently opening its contacts.

Another weird problem reported is the PCM Power Diode being loose in its slot. Check its security. The Diode routes powers to the PCM relay.

The PCM relay also controls power to the fuel pump relay coil. So if the PCM relay opens, spark and fuel are both lost.

 

[mridenour]

Where is the inertia switch located? I can check all these things this afternoon. Thanks for your help.

 

[Earl43P]

Above where the passenger's toes hit, behind the carpet.

 

[Toolguy]

1994 Ranger. 234,193 miles. 4.0 manual trans. Parts replaced in the last 6 months: fuel pump (twice), fuel pressure regulator, fuel filter, TPS, IAC, MAF, engine coolant sensor, both O2 sensors, plugs and wires.

The fuel pump went bad back in Nov of 2010. After replacing it along with the TPS, IAC, MAF, plugs and wires the truck ran fine for approx 150 miles then went back to what it was doing back in Nov. I thought the new fuel pump had gone bad, so still under warranty I got another one and installed it, no effect this time. When first cranked from cold it runs fine for about 10 minutes, and then seems to just die like it is starving for fuel. The idle will drop from approx 1000 rpms to 500 rpms. When the accelerator is pressed it wants to completely die. On occasion while driving there are times when I can feel the engine pick up like it is supposed to, its almost like something either makes contact or normal fuel pressure is restored to the injectors but it only lasts for a minute or so then back to no power and barely running. I am stumped.

5/31--compression check told the story, rebuild time. First comp stroke 90 building over 5 successions to approx 180.

 

[mridenour]

Tried a bunch of the stuff that was suggested and nothing changed. Unplugged the MAF and the truck runs a million times better. We cleaned the MAF previously and it didn't help. What is this telling me now? Replace MAF or is there a component screwing with the computer?